The present embodiments relate to an arrangement of equipment with a microwave generator and a pulse generator connected to the microwave generator via a transmission element.
To generate electromagnetic fields in the microwave range (e.g., at a frequency in the range between approximately 0.3 and 300 GHz), microwave generators in the form of space-charge-wave tubes (e.g., a magnetron or klystron) may be used. A space-charge-wave tube such as this enables outputs in the megawatt range to be achieved (e.g., by supplying the space-charge-wave tube with a pulsed supply current). Such fields are used, for example, in electron linear accelerators, as are used in medical engineering, to accelerate an electron beam.
With an arrangement of equipment of this type, it is, for example, a complex matter to implement an effective transmission of the current pulses from the pulse generator to the microwave generator.
The reason for this is that the electrical properties of a space-charge-wave tube (e.g., a magnetron) may change abruptly if the supply voltage (e.g., a break-through voltage) exceeds a threshold. A close approximation of the circuit engineering behavior of a magnetron may be described by an equivalent circuit diagram, in which a switch with a downstream ohmic resistance is connected in parallel to an attenuated series resonant circuit. The operation of the magnetron at a supply voltage undershooting the break-through voltage corresponds to a state of the equivalent circuit diagram, in which the switch is open. In this operating state (e.g., standby mode), the magnetron essentially acts as a series resonant circuit in the supply current circuit.
Exceeding the break-through voltage corresponds in the equivalent circuit diagram to closing the switch. In this second operating state or operating mode, the magnetic current essentially acts in the supply current circuit as an ohmic resistance having an impedance that is low-resistance compared to the impedance of the series resonant circuit.
In pulse operation of a magnetron, the break-through voltage is temporarily exceeded with every current pulse or every voltage pulse, which in the equivalent circuit diagram, equates to a temporary closure of the switch. Every time the break-through voltage is exceeded or undershot, the result is an abrupt change in the load, which the magnetron represents in the supply current circuit. These abrupt changes in load or impedance result, in the absence of special precautions, in undesired fluctuations in the time characteristic of the supply current that supplies the magnetron.
In order to suppress these current fluctuations as much as possible, impedance matching of the supply current circuit may be performed. Impedance matching may be effected by connecting a dynamic load (e.g., a capacitor) in parallel to the magnetron.
One disadvantage of this solution is that the components connected into the supply current circuit for impedance matching are to be individually aligned with the other components of the supply current circuit (e.g., the pulse generator and the magnetron) that significantly impedes a flexible design of the structure. Accordingly, one of these components may not be replaced (e.g., a given individual magnetron or a given individual pulse generator) without again performing impedance matching. The components aligned with one another may not be moved without impairing the impedance matching (e.g., if, as a result, the transmission path (the length of the transmission element) is decreased or increased).
In addition or alternatively to separate circuit components for impedance matching, an RC network (e.g., attenuation network) may be connected in parallel to the magnetron and limits the voltage rise to a value specified by the manufacturer for the magnetron in question. Thanks to the attenuation network, an excessively fast voltage rise, to which the magnetron would respond with tube surges and malfunctions on commencement of the oscillation, may be prevented. By correctly adjusting the attenuation network, an excessively slow voltage rise, which would not result in the commencement of the oscillation and which would cause increased losses in the tube, may be avoided. However, the parts for the attenuation network generate additional costs and account for a certain risk that the magnetron will fail during operation.